The present invention relates in general to characterization and mapping techniques of a semiconductor substrate using photocurrent measurements induced by a laser beam scanning the surface of the semiconductor substrate. The photocurrents are collected through contacts established with an electrolytic solution. More particularly, the present invention relates to characterization techniques of an interface between a semiconductor substrate and a thermally grown or deposited dielectric layer.
Photocurrent measurements using an electrolyte in contact with the surface of the semiconductor substrate, and in which is immersed an electrode of a nonpassivating noble metal, are notably used to measure the mean life of bulk minority carriers of a semiconductor silicon wafer, as disclosed in Lehmann et al., J. Electrochem. Soc. 135, 2831 (1988). A nonpassivating noble metal, such as platinum, establishes electrical connection to a biasing circuit. According to this technique, excess carriers are injected using a laser beam on the wafer""s front surface and are collected through an inversely biased Schottky contact. The Schottky contact is formed on the wafer""s back surface (backside photocurrent mode) or on the same wafer""s front surface (frontside photocurrent mode).
Often, when operating in a backside photocurrent mode by collecting the photocurrent at the wafer""s back surface, an area of the wafer""s front is immersed or contacted with a solution suitable to passivate the superficial layers of the wafer. Also, the Schottky contact is biased through a contact established on the wafer""s back surface using an electrolyte with a nonpassivating electrode of a noble metal, such as platinum, immersed in the electrolytic solution.
The Schottky contact must be inversely biased with respect to the bulk potential of the wafer. The wafer is grounded through one or more resistive contacts formed on the silicon around the contacted areas. The one or more resistive contacts may also be formed through the electric charges collecting electrolytic solution, and eventually through the passivating solution.
In commercially available instrumentation, such as, for example, the Electrolytic Metal Tracer (Elymat) of GeMeTec GmbH, both the passivating solution and the electrolytic solution used to form the Schottky contact for collecting photocurrents includes a diluted hydrofluoric (HF) acid solution. Any native or thermal oxide film that may be present on the surface of the semiconductor wafer is etched away by the HF solution. This also ensures an optimal surface passivation of the silicon, as disclosed in Yablonovitch et al., Phys. Rev. Letters 57, 249 (1986).
Based upon the intended application, the current may be collected on either the wafer""s backside or frontside through electrodes of a nonpassivating noble metal, as disclosed in Foll, Symp. on Advanced Science Technology and of Silicon Material, Kona, Hawaii, 1991. The nonpassivating noble metal typically includes platinum.
The light source for stimulating the photocurrent is a laser beam focussed on the wafer""s surface, and eventually through the film of a passivating HF solution. The laser beam is scanned in successive lines to produce maps of the mean life of the bulk carriers.
Proposals have been made for modifying this measuring technique to adapt it for evaluation of the surface recombination velocity at the interface between a semiconductor substrate and a dielectric layer. The substrate is typically silicon, and the dielectric layer is a thermally grown silicon oxide, i.e., a gate oxide or a tunnel oxide, for example. This is disclosed in the following articles: Polignano et al. Journal of Solids non-Crystalline 216, 88 (1977), Ostendorf et al., Defects and Impurity Engineered Semiconductor Devices PV 378, p. 579, The Material Research Society Symposium Proceedings, Pittsburgh, Pa (1995).
According to these modifications, the laser -beam scans the surface of the dielectric layer that covers the semiconductor substrate in absence of a film of a passivating acid solution. Otherwise, if present, it would etch the dielectric oxide and would destroy the interface being examined.
In these conditions, the photocurrent collected through the Schottky contact depends on the bulk minority carrier""s mean life as well as on the surface recombination velocity at the semiconductor-dielectric interface. The Schottky contact is preferably established on the backside of the wafer of semiconducting silicon through an electrolyte. Both parameters may be assessed from a sequence of measurements carried out at different conditions.
The surface recombination velocity is a parameter that may be extremely useful for characterizing semiconductor/dielectric interfaces. For example, an oxide/silicon interface of a gate oxide or tunnel oxide may be characterized.
The extension of photocurrent measuring techniques to the characterization of semiconductor/dielectric interfaces is of great interest because these measurement techniques, as compared to others, require a minimum sample preparation and are able to generate detailed maps of the entire wafer""s surface. Although of great interest, this technique has serious limitations. The surface recombination is significantly influenced by the density of the superficial layers and by the surface potential.
It has been demonstrated that the surface recombination velocity may dramatically vary upon the varying of the surface potential, as disclosed in Aberle et al., J. Appl. Phys. 71, 4422 (1992). The surface potential varies from a condition of an accumulation of majority carriers with respect to the bulk density, to a condition of inversion. The interface at the silicon is a concentration of minority carriers comparable to the concentration of majority carriers in the bulk, as it may occur in presence of electric charges in the oxide at the interface.
However, according to the present techniques, the surface potential cannot be controlled and effects due to the superficial layer density and to eventual electric charges in the oxide cannot be distinguished.
The shortcomings of the above described techniques are overcome by substantially forming a gate electrode using a layer of electrolyte in contact with the dielectric layer and biased through an electrode immersed in it. This is made possible by using an electrolyte that is not aggressive to the dielectric oxide. A solution of an organic acid, such as an acetic acid solution, for example, and biased with a platinum electrode is suitable to form an effective gate electrode coupled to the silicon oxide dielectric. Through this interface it is possible to control the surface potential at the semiconductor/dielectric interface.
The possibility to control the potential at the semiconductor/dielectric interface by suitably biasing the gate electrode formed by the electrolyte, with respect to the semiconductor substrate potential, permits assessment of the surface recombination velocity at the interface as a function of the voltage applied to the gate electrode. This is in addition to a charge injection level established by controlling the scanning laser beam. The analysis of this data permits determination of both the density of surface states and the electric charges in the dielectric. An analytical approach similar to the one described above may be utilized for a quick evaluation. A more accurate analysis may be carried out by the use of a numerical processor for processing device equations. Device equations include, for example, continuity and drift-diffusion equations.
Compared to the known methods of measuring the surface recombination velocity, the method of the invention has the advantage of discriminating between the effects caused by density of surface states and by eventual electric charges in the dielectric, thus providing information comparable to that obtained through capacitance-voltage measurements. Compared to traditional characterization methods based on capacitance-voltage measurements, a method based on surface recombination measurements has the advantage of not requiring the formation of capacitors, and therefore, permits faster quality control checks of the semiconductor/dielectric interface.
Moreover, the wafers may be more accurately mapped through surface recombination measurements than through capacitance-voltage measurements, and an accurate map of the entire wafer may often reveal the reasons for an observed interface degradation.